The field of the invention is the synthesis of ammonia in a continuous process whereby a gas mixture containing an approximately stoichometric ratio of hydrogen and nitrogen is passed over a series of catalyst beds at relatively high pressure and controlled temperatures. Specifically, the invention herein relates to temperature regulation in this process by means of heat exchange effected between portions of the gas mixture itself at various stages of its progress through the process.
Ammonia production as commercially practiced utilizes the seemingly straightforward reaction between nitrogen and hydrogen in stoichometric amounts: N.sub.2 +3H.sub.2 .fwdarw.2NH.sub.3. The reaction is exothermic; accordingly, the equilibrium is shifted to the right by lower temperatures. However, as a practical matter, the temperature must be maintained at an elevated level in order to increase the reaction rate sufficiently to carry out the process in a reasonably short amount of time, even though catalysts are also used to accelerate the rate of the reaction. Thus, an appropriate balance between thermodynamic and kinetic considerations determine the appropriate temperature range at which the synthesis should be operated.
Thermodynamic considerations would also millitate that the reaction would be favored by higher pressure, since collisions between gas molecules are required to effect the synthesis. The pressure range at which this process is generally carried out is over 100 atmospheres, although it has been disclosed that synthesis procedures are possible with pressures of as low as 20 atmospheres (U.S. Pat. No. 3,957,449).
Temperature regulation is most often accomplished by a "quench" type ammonia conversion process. In this process, the synthesis gas containing nitrogen and hydrogen in roughly stoichometric amounts (syngas), preferably with as few diluents as possible, is passed through a catalytic bed of, for example, iron or promoted iron, to produce an effluent which is at a higher temperature than the original mixture due to the exothermic nature of the reaction. The effluent contains some percentage of ammonia, representing for example, 10 to 15% total volume. The temperature of the emerging gas is ordinarily sufficiently high to be thermodynamically inhibitory to further reaction. Therefore, before the effluent is passed through still another catalyst bed in order to increase the percentage conversion to ammonia, it is mixed with "cold" fresh synthetic gas thus lowering the temperature of the new mixture to the proper level. This process may be repeated for as many passes through catalyst beds as is desired. However, it suffers from the drawback that obviously not all of the syngas will pass through all of the catalyst beds.
U.S. Pat. No. 4,230,680 to Becker describes an alternative process whereby rather than mixing fresh syngas with partially converted effluent, only heat exchange between the fresh syngas and effluent is effected, not physical mixing of the gases. In the Becker process, a portion of effluent from each and every catalytic bed in the series is passed through a heat exchanger in which portion of the feed syngas provides a heat sink. U.S. Pat. No. 3,851,046 to Wright and Pickford discloses a two-bed process in which heat exchange is effected between effluent from the first bed and fresh syngas and the effluent from a single second bed is cooled by high-pressure steam generation. Both of the foregoing approaches turn out to be less efficient than that of the present invention wherein only the effluent from the first pass of syngas over catalyst is heat exchanged; and further cooling of subsequent effluents from multiple beds is accomplished by high pressure steam generation, which high pressure steam may then be employed for other purposes.